Dilute hydrochloric acid induced switching from antagonism to synergism in the binary mixed systems of cocamidopropyl betaine (CAPB) and sodium dodecyl benzene sulfonate (SDBS)

Abstract

Abstract A synergism is reported in the binary mixtures of cocamidopropyl betaine (CAPB) and sodium dodecyl benzene sulfonate (SDBS) at mole fractions of 0.25, 0.50, and 0.75, in dilute hydrochloric acid solutions (∼0.05 M) HCl, at 25 °C. Such synergism is confirmed by surface tension values, critical micelle concentration (CMC), and interaction parameter values according to Holland and Rubingh’s model and the satisfaction of Hua and Rosen conditions. Surface excess (Γ) at air/water interface and the minimum area occupied by surfactant molecule (A min) also indicated a reduction in the A min of mixed CAPB/SDBS compared to single surfactants that is due to the strong electrostatic interaction between surfactants heads. The composition of CAPB/SDBS mixed micelles, the interaction parameter (β) and the micellar activity coefficients (f 1 and f 2) were evaluated from the regular solution model for these binary amphoteric/anionic mixed systems. It is clear the CAPB surfactants are converted to the cationic form by charging in the presence of HCl, which enhances the interaction between the oppositely charged CAPB and SDBS heads and overcomes the unfavourable packing between the tails predicted in pure water. Such strong interaction was also reflected on the phase behavior of CAPB/SDBS binary mixed system. Accordingly, a switching from antagonism to synergism is reported for CAPB/SDBS binary mixed system in the presence of dilute hydrochloric acid.